1. Technical Field
This invention applies to turbine engine rotor assemblies in general, and to apparatus for sealing between adjacent rotor blades within a turbine engine rotor assembly in particular.
2. Background Information
Turbine and compressor sections within an axial flow turbine engine generally include a rotor assembly comprising a rotating disc and a plurality of rotor blades circumferentially disposed around the disc. Each rotor blade includes a root, an airfoil, and a platform positioned in the transition area between the root and the airfoil. The roots of the blades are received in complementary shaped recesses within the disc. The platforms of the blades extend laterally outward and collectively form a flow path for the fluids passing through the turbine. A person of skill in the art will recognize that it is a distinct advantage to control the passage of fluid from one side of the platforms to the other side of the platforms via gaps between the platforms. To that end, it is known to place a seal between the blade platforms to control such fluid leakage.
During the operation of the turbine engine, air flow on the airfoil side of the platforms (generally referred to as "primary flow") is at a significantly higher temperature than airflow passing by on the root side of the platforms (generally referred to as "secondary flow"). The high temperature primary flow, the temperature gradient across the platform, and the lack of platform cooling in most blade designs combine to produce high thermal stresses within the platforms which can cause stress cracks. To alleviate the stress, it is known to bleed the lower temperature secondary flow through small apertures within the platform. This solution does help to reduce the thermal gradients across the blades and therefore reduce the thermal stresses within the platforms. There is a limit, however, to the amount of leakage that may pass through the platforms using this method.
Upstream of the turbine stages of the engine, work imparted to the secondary flow by the compressor stages of the engine increases the pressure of the secondary flow. Passing secondary flow through platform apertures loses some of that imparted work and therefore decreases the efficiency of the engine. To minimize the loss of work while optimizing the cooling done by the secondary flow, it is known to use a greater number of smaller diameter apertures, rather than a fewer number of larger diameter holes. Decreasing the diameter of the hole, however, increases the stress concentration about that hole. Hence, there is a tension between the benefits of cooling and the detriments of cooling holes using the aforementioned method.
In sum, what is needed is a means for sealing between adjacent rotor blades in a turbine engine rotor assembly which alleviates the formation of thermal stress within the blade platforms and which does not appreciably reduce the efficiency of the engine.